The left wing of the aircraft separated in flight. Evidence on the left wing rear attachment indicates that the retaining pin disengaged in flight and that it was not secured by a lock ring. This evidence indicates that the pin could move freely and that it disengaged gradually, probably over the course of several flights. The other wing attachment points showed evidence that they had failed in overload following excessive movement of the wing. Because the aircraft owner did not have to assemble the wings before each flight as the previous owner had, the condition of the attachments was not checked before each flight. It is clear that a pre-flight inspection of the left wing attachment was not performed. The modification to this attachment did not conform to the drawings submitted by the designer of the aircraft. Although the modification was not suitable for the wing attachment, it does not seem to have contributed to the wing separation.Analysis The left wing of the aircraft separated in flight. Evidence on the left wing rear attachment indicates that the retaining pin disengaged in flight and that it was not secured by a lock ring. This evidence indicates that the pin could move freely and that it disengaged gradually, probably over the course of several flights. The other wing attachment points showed evidence that they had failed in overload following excessive movement of the wing. Because the aircraft owner did not have to assemble the wings before each flight as the previous owner had, the condition of the attachments was not checked before each flight. It is clear that a pre-flight inspection of the left wing attachment was not performed. The modification to this attachment did not conform to the drawings submitted by the designer of the aircraft. Although the modification was not suitable for the wing attachment, it does not seem to have contributed to the wing separation. The left wing attachment had been modified by the builder and was not in accordance with the design drawings. The retaining pin was not secured by a lock ring. The retaining pin disengaged and allowed the left wing to twist, causing the other wing attachments to fail in overload. The left wing separated in flight. The left wing attachment was not visually inspected prior to the flight.Findings The left wing attachment had been modified by the builder and was not in accordance with the design drawings. The retaining pin was not secured by a lock ring. The retaining pin disengaged and allowed the left wing to twist, causing the other wing attachments to fail in overload. The left wing separated in flight. The left wing attachment was not visually inspected prior to the flight. The retaining pin was not secured by a lock ring and it disengaged in flight. A pre-flight inspection was not performed.Causes and Contributing Factors The retaining pin was not secured by a lock ring and it disengaged in flight. A pre-flight inspection was not performed.